Burners ignite a fuel, such as natural gas or heating oil to produce heat. Modern burners typically have one or more sensors capable of monitoring the presence of a smaller ignition flame (e.g., a pilot light) as well as the larger flame that produces the heat. A variety of sensors are known, and they range in functionality (and price) as the use of the burner dictates. For example, high-end flame sensors may analyze the performance of the flame spectroscopically, and alert engineers or service technicians when the burner is not functioning optimally.
Many burners rely on flame rectification to sense the presence of a flame. In flame rectification an AC signal is sent to two electrodes contacted by the flame. Because the current from one half of the AC cycle is preferentially conducted through the flame, the AC signal is rectified to produce a noisy DC voltage. Thus, by observing a heavily-filtered DC voltage, it is possible to determine if the flame is present. If the DC voltage falls below a pre-determined value, the sensor assumes that the flame is out, and a valve shuts off the fuel flow. Flame rectification cannot distinguish between a high-resistance flame and no flame, however, because either condition results in a DC voltage below the acceptable level.
Because of the harsh environment inside the flame, the electrodes for flame rectification must be fabricated from high temperature alloys, such as kanthol, or ceramic material. These electrodes, intended for exposure to flames, are known generally as flame rods. Flame rods are available from a number of manufacturers including, but not limited to, Honeywell Inc. (Golden Valley, Minn.). In many embodiments, the burner or pilot light assembly is the second electrode needed for flame rectification, thus, typically, only one flame rod is necessary to monitor a flame with flame rectification.
Over time, flame rods develop a layer of corrosion, soot, carbonization, contamination, etc. This contamination layer may negatively affect the function of the flame rod by increasing the resistance of the flame rod. As the contamination builds up, the observed DC voltage will decrease to the point that a flame-out (no flame) event is registered. In many cases, the flame-out event will trigger an unnecessary shut-down of the burner. To avoid unnecessary shut downs, flame rods must be serviced regularly to verify that they do not have a contamination layer that will trigger a flame-out event.